The present invention relates to digital time-division-multiplex (TDM) communication systems, and, more particularly, to multi-party conference circuits for providing multi-party telephone calls in digital TDM communication systems.
In digital TDM communication systems, samples of the speech signals are taken at a periodic rate, for example, 8 KHz, then quantized and time multiplexed onto channels of a digital highway in preassigned time intervals. The time interval between the taking of samples is commonly referred to as a frame. The number of channels allocated to a frame is dependent upon the particular type of TDM communication system utilized.
Most commercially available digital TDM communication systems utilize pulse-code modulation (PCM), the characteristics have been standardized by the CCITT in sections Q.46 and Q.47 of the Green Book, Volume 3, Section 7, entitled "Digital Transmissions Systems", published by the Fifth Plenary Assembly at Geneva, 1972. The two PCM formats specified by the CCITT are the 24-channel format wherein a frame contains 24 channels, and the 32-channel format, wherein a frame contains 32 channels. The digitized voice samples in these PCM systems are eight bits in length, and are derived according to non-linear companding codes, which are referred to as the .mu.-law for the 24-channel format and the A-law for the 32-channel format. Groups of 24 channels for the 24-channel format or 32 channels for the 32-channel format may be time division multiplexed onto parallel TDM highways to facilitate processing within the digital switching portion of the PCM communication system.
In the digital switching network of these PCM communication systems, two-party conversations are readily provided by simply exchanging digitized speech signals between the respective two channels of the parties. However, problems arise when attempting to provide a multi-party conversation.
Various schemes for providing multi-party conference calls have been utilized in the prior art. For example, some prior art systems, such as those described in U.S. Pat. Nos. 3,699,264, 4,007,338, 4,022,981, 4,022,991, and 4,126,766, transmit to the other parties the speech signals from the party whose speech signal has the largest magnitude, or in other words, the party speaking the loudest. Other prior art systems, such as those described in U.S. Pat. Nos. 3,984,643, 4,109,111, and 4,119,807, essentially accumulate the sum of the speech signals from all parties during a frame, and then transmit to the parties a combined speech signal formed by subtracting the speech signal for the respective party from the accumulated sum of the speech signals for all parties. In yet other prior art systems, such as those described in U.S. Pat. Nos. 3,924,082, 4,054,757, and 4,059,735 , the speech signals for a group of N parties are stored, after which combined speech signals for each different combination of N-1 parties are totallized and transmitted to the corresponding parties. Thus, the foregoing prior art systems either provide the loudest speaker to all parties; or over each frame, totallize and store the speech signals of all parties and then provide to each party the totallized speech signals less the respective party's contribution; or, over each multi-party grouping of channels, store the speech signals for all parties and then totallize the speech signals from the other N-1 parties for transmission to each party. In most of these prior art systems, a frame or more of time delay is introduced into the speech path, since the speech signals from all parties must first be stored before processing can be completed. Moreover, none of the prior art systems totallize the newly received speech signals and transmit the totallized speech signals to the corresponding parties in a time interval of less than one frame, without making use of an intermediate storage unit to accommodate speech signals of all N parties.
Accordingly, it is an object of the present invention to provide an improved three-party conference circuit for providing conference calls between groups of three parties, wherein the speech signals received from each party are combined and transmitted to the respective parties as they are received.
It is another object of the present invention to provide an improved three-party conference circuit for providing conference calls between groups of three parties, that may be advantageously utilized in various TDM communications systems having different channel capacities and different companding codes for digitizing speech signals.
It is yet another object of the present invention to provide an inexpensive three-party conference circuit for providing conference calls between groups of three parties, while also providing capability for converting between different companding laws, looping back speech signals for diagnostic purposes, and broadcasting a pre-selected channel in combination with all other channels of the frame.